Thermal and Humidity Effects on Electronic Equipment

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De-lamination

De-lamination of composite materials may have a devastating effect on some cheap PCB base materials.

Dimensional changes

This can result in not only high stress because of bowing/twist but also swelling of fibrous materials.

Fibres

When exposed to humidity, fibres with high moisture-regain values usually not only lose tensile strength and extensibility on the absorption of water but also suffer a fair degree of swelling.

Surface resistivity

On PCBs, reduction in surface resistivity can alter timing circuits (which changes the frequency of oscillator circuits), change the current level in a constant-current source, result in loss of sensitivity, or reduce the input impedance on high-impedance amplifiers.

Metal migration

Metal migration, especially silver migration, occurs between closely spaced conductor lines in the presence of moisture and an applied voltage. It causes reduction of insulation resistance, increase in current leakage and eventually catastrophic shorting. Metal migration is the cause of many microcircuit failures.

Fig. 2: Metal migration—water-drop test for a silver-platinum, thick-film conductor (complete bridging after 25 minutes, 4V DC)
Fig. 3: Metal migration—water-drop test for thick-film gold conductor (no evidence of metal migration after 1 hour, 4V DC)

Outgassing from PCB laminate

Many PCBs assembled by a number of different commercial companies have experienced blow holes or voids in solder joints due to outgassing from resins. These defects can be traced to:

1. Moisture. One of the main causes of outgassing during the soldering process is release of moisture (absorbed by the resin during long-term storage prior to assembly) in gaseous form due to the extreme temperature during soldering, resulting in poor solder joints, pinholes, blow holes or rupturing of plated-through holes.

2. Multilayer technology. The rupturing of a plated-through hole in multilayer boards can also create such problems as disconnection of internal tracks and the resultant high rework costs.

Secondary effects

In the electronics industry, poor hygiene standards may add sufficient contamination to supply the necessary nutrients to support growth formations.

Contamination from the following sources can cause long-term damage that may not be detected during inspection or test:

PCB manufacture

Contamination may take place during actual board production and is usually due to either incomplete curing of the resin or badly applied solder resist.

No-clean fluxes

The input impedance of devices can also get affected by the presence of humidity vapour on any polar contamination in the flux residue. When using hand-soldering techniques, some modern, synthetic no-clean fluxes will not reach deactivating temperature of the flux. If subjected to high humidity for a period of two to three weeks, the result is a white organic salt residue which, although low in ionic contamination, can easily trap moisture. This, in turn, can affect surface resistivity. Hydroscopic nature of these salts may further compound the problem as these present a perfect medium to precipitate dendritic growth when power is applied to the PCB.

Electrolytic corrosion

Usually, corrosion requires presence of moisture and soluble impurities, which may be inherent in the materials or contained in atmospheric pollution. These provide the electrolyte necessary for electro-chemical reaction of the corrosion process. The reaction takes place where dissimilar metals are in contact or may also be initiated when an electrolyte bridges the gap between metal surfaces and current flows. It is not necessary to have visible wetting of the surface; an invisible adsorbed film of moisture is sufficient to support electrolytic corrosion.

The presence of conductive electrolyte on the surface of PCBs under voltage stress can result in surface tracking because of corrosion or, in the worst case, electro-migration of metal across the gap resulting in a short circuit.

The ingress of moisture into a connector may result in corrosion around pins. Corrosion in connectors can increase surface resistance with the possibility of over-heating culminating in loss of performance or even a fire.

Low humidity problems

Most of the low humidity effects are evident in hydroscopic materials. In some cases, moisture removal can change the mechanical structure and cause embrittlement. Often, this is also accompanied by shrinkage and weight loss.

Static damage

One major difficulty with static damage is that the results are not always immediately obvious because the resulting failure often destroys the evidence of damage. It is therefore difficult to categorically state that the failure was caused by static damage. The most critical period when damage can be inflicted is handling before a device is inserted into the board.

Dust

Static charge attracts dust. The buildup of dust in an unsealed unit can result in malfunction by causing bad contacts or creating a path that allows tracking of high-voltage discharges to the earth—such as in a TV or computer monitor.

Electrostatic generation

Static charge generation is greatly affected by humidity levels. Voltages as high as 20kV can be generated by a person walking across a carpet when humidity levels are below 30%RH. Under high humidity conditions, the same walk may generate only about 1.5kV. If this person picks up a device without following the correct antistatic measures, device damage can occur.

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